1. Field of the Invention
The present invention relates to a method and a device for driving a direct current (DC) brushless motor wherein excitation current is supplied to the drive windings under duty ratio control, the rotational position of the rotor is detected based on voltages induced in the drive windings owing to rotor rotation, and commutation control is effected in accordance with the detected position.
2. Background Art
The prior art includes a DC brushless motor driving device which achieves so-called "sensorless operation" by effecting the required commutation control based on the rotational position of the rotor detected from voltages induced in the drive windings by rotor rotation. A configuration is also known for regulating the average voltage applied to the drive windings of a DC brushless motor by varying the duty ratio of a chopping signal. When an attempt is made to effect sensorless operation in a configuration that chops the voltage applied to the drive windings, however, the sensorless operation is hindered by the superimposition of the pulse signal for chopping on the voltage induced in the drive winding. Japanese Patent Application Publication No. Sho 58-25038 ('038) teaches a configuration for overcoming this difficulty by separating the two types of signal components with filters so as to enable sensorless operation to be conducted unaffected by the chopping signal.
The teaching of '038 is to obtain the voltages induced in the drive windings through first-order lag filters to remove the chopping frequency component associated with the duty ratio control, thereby obtaining signals wherein the timing of polarity inversion of the voltage level induced in the open phase is phase-delayed by 90.degree., and to use these signals for controlling the commutation of drive currents to the drive windings.
In an actual DC brushless motor driving device, however, other phase lag factors are also present in addition to the phase lag produced by the first-order lag filters. For instance, a certain time lag not dependent on the rotational frequency occurs, while the inductance components of the drive winding coils delay the rise of the motor current after current switching. These phase lag factors produce commutation lag that tends to become particularly pronounced and degrade motor efficiency in the high speed region. Moreover, since effecting commutation with the timing of the polarity inversion delayed by 90.degree. as taught by '038 results in commutation by use of signals for one phase earlier, the control is not optimum with respect to motor rotation and, particularly in the low speed region where inertial force is small, is likely to cause fluctuating rotation and vibrational noise.
To cope with these problems, Japanese Patent Application Public Disclosure No. Hei 8-80083 teaches a digital control method that enables the commutation control to be effected at a timing delayed by 30.degree.. Since this digital method can discriminate the induced voltages in the drive windings only when the chopping signal is on, however, it cannot always effect commutation with optimum timing. To avoid this problem, the chopping frequency must be made variable, but this requires complex, high-cost circuitry.